Combustion engines such as diesel engines, gasoline engines, and gaseous fuel-powered engines are supplied with a mixture of air and fuel for combustion within the engine in order to generate a mechanical power output. Such combustion engines exhaust a complex mixture of air pollutants as byproducts of the combustion process, and due to increased attention on the environment, exhaust emission regulations continue to become more stringent. The amount of pollutants emitted to the atmosphere from an engine can be regulated depending on the type of engine, size of engine, and/or class of engine. For example, the International Maritime Organization (IMO) has designated various emission zones off of the coasts of North America and Europe. Effective in 2014, the IMO's emissions regulations (IMO3) will be most stringent in a first zone extending approximately 200 miles from the coast of these continents, and will be less stringent in a second zone extending beyond approximately 200 miles from such coastal areas.
One method that has been implemented by engine manufacturers to comply with the regulation of exhaust emissions includes utilizing an EGR system. EGR systems operate by recirculating a portion of the exhaust produced by the engine back to the intake of the engine to mix with fresh combustion air. The resulting mixture has a lower combustion temperature and, subsequently, produces a reduced amount of regulated pollutants. Although such EGR systems may be useful in reducing the amount of regulated pollutants produced during combustion, utilizing EGR can adversely affect, for example, the air compressing capabilities of the engine's air induction system. For example, diverting a portion of the combustion exhaust to the EGR system may cause one or more turbochargers associated with the engine to operate below its peak efficiency range. Thus, the reduced air compressing capability caused by activation of the EGR system may reduce the engine's fuel economy and, possibly, the amount of power generated by the engine.
An example of a turbocharged engine utilizing EGR technology is disclosed in U.S. Pat. No. 6,694,736 (the '736 patent) issued to Pfüger on Feb. 24, 2004. The '736 patent discloses an engine with a common intake manifold and divided exhaust manifolds. Two high-pressure turbochargers having respective high-pressure compressors and high-pressure turbines are separately associated with the common intake manifold and the two exhaust manifolds, and a single low-pressure turbocharger receives exhaust from each of the two high-pressure turbochargers. In addition, exhaust return pipes are connected to the intake manifold downstream of the high-pressure compressors to direct exhaust from upstream of the high-pressure turbines back into the engine.
While the system disclosed in the '736 patent may utilize EGR technology to reduce the amount of pollutants present in the engine exhaust, the disclosed system may not be well suited for all applications. For example, operating the engine of the '736 patent and its EGR system to satisfy relatively stringent emissions regulations (such as those discussed above with respect to IMO3) may decrease the amount of exhaust directed to the turbochargers. Such operation may cause one or more of the turbochargers to operate outside of its peak efficiency range, thereby resulting in lower fuel economy and less than optimal power generation.
The disclosed exhaust system is directed to overcoming one or more of the problems set forth above and/or other problems of the prior art.